In the following, the magnitude of an underpressure in the tank-venting system is discussed. Here, a "high" underpressure refers to an underpressure having a high absolute value. Correspondingly, a threshold underpressure is then exceeded by the actual underpressure when the absolute value of the actual underpressure is greater than the absolute value of the threshold underpressure.
U.S. patent application Ser. No. 08/070,334, filed May 26, 1993, is incorporated herein by reference and discloses a method and an arrangement for diagnosing a tank-venting system wherein the decay gradient for the underpressure in the tank-venting system is used. The tank-venting system includes: a tank having a tank-pressure sensor; an adsorption filter which is connected to the tank via a tank-supply line and which has a venting line closeable by a shutoff valve; and, a tank-venting valve which is connected to the adsorption filter via a tank-venting line. The shutoff valve is closed to produce underpressure and the tank-venting valve is opened. The tank-venting valve is again closed as soon as a pregiven underpressure has developed. In this way, the underpressure decays and does so relatively slowly when the tank-venting system is tight. However, the underpressure decay gradient is not only dependent upon the tightness of the system, but also on the fill level in the tank. This influence can, however, be substantially eliminated when the underpressure build-up gradient is measured and the two gradients are set into relationship with each other. This is disclosed in U.S. patent application Ser. No. 08/129,039, filed Oct. 4, 1993 and incorporated herein by reference.
For the build-up of underpressure, the tank-venting valve is opened in the methods known to date at a pregiven maximum pulse-duty factor. This pulse-duty factor is so selected that the desired underpressure is not reached before a desired minimum time span has elapsed even for a tank which is almost full. This time span is especially selected in dependence upon the time-dependent detection of other measured variables such as a lean-correction measured variable. If the tank is relatively empty, the time span for pumping out the tank-venting system to the desired underpressure is considerably lengthened because of the fixed pulse-duty factor. This lengthening operates directly on the time span which is necessary for the overall checking sequence. A lengthening of this time span is, however, critical because a tank-venting diagnosis with an evaluation of the underpressure decay gradient can only supply reliable results during specific operating conditions of a motor vehicle. Idle is especially preferred. Idle phases typically last only several 10 second increments. For this reason, the check time span cannot be too greatly increased by the time span required to reach the desired underpressure. It should here be noted that for a tank having a holding capacity of 60 liters and a pulse-duty factor of 10% at idle, a 2.5 liter four-cylinder spark-ignition engine requires approximately 40 seconds until an underpressure of 5 hPa is reached.